1. Field of the Invention
The present invention relates to digital signal encoding and decoding methods, apparatuses for executing these methods, and storage media storing relevant executing programs.
This application is based on Patent Application No. Hei 9-161152 filed in Japan, the contents of which are incorporated herein by reference.
2. Description of the Related Art
In the coding of digital signals, the amount of generated codes is controlled by adjusting the width for quantization (i.e., quantization width, or the number of quantization steps). If the amount of actually generated codes is larger than a target amount of codes, then the amount of generated codes is depressed by increasing the quantization width. On the other hand, if the amount of actually generated codes is smaller than a target amount of codes, then the amount of generated codes is increased by narrowing the quantization width.
According to each objective for coding of digital signals, various units may be used for changing the quantization width. For example, in the method "ISO/IEC 11172-2 (MPEG-1)" of coding moving pictures, the quantization width can be determined for each small block of 16.times.16 pixels. If the quantization width is changed for each specific unit and information of the quantization width is transmitted for each unit, then the amount of codes necessary for transmitting the information becomes large and thus efficiency of coding is lowered. Therefore, more desirable methods are known such that (i) information of the quantization width is transmitted only when the width is actually changed; and (ii) a difference between the quantization width of a signal to be encoded next and the quantization width of an already-encoded signal is transmitted.
As another method for depressing the amount of codes for transmission of the quantization width, the variable range of the quantization width may be restricted so as to shorten the length of codes which indicate the quantization width. According to this method, the amount of codes necessary for transmitting the quantization width can be depressed and simultaneously the quantization width can be changed.
In the above conventional techniques, information indicating the quantization width must be transmitted every time the quantization width is changed, and increase of the amount of codes according to this transmission is inevitable.
In addition, the variable range of the quantization width is restricted; thus, even if the quantization width is maximized within the range, the amount of generated codes may not approach the target amount of codes.